Our Changing Climate

Future Changes in Global Climate

Earth’s climate will continue to change over this century and beyond. Past mid-century, how much the climate changes will depend primarily on global emissions of greenhouse gases and on the response of Earth’s climate system to human-induced warming. With significant reductions in emissions, global temperature increase could be limited to 3.6°F (2°C) or less compared to preindustrial temperatures. Without significant reductions, annual average global temperatures could increase by 9°F (5°C) or more by the end of this century compared to preindustrial temperatures.

Beyond the next few decades, how much the climate changes will depend primarily on the amount of greenhouse gases emitted into the atmosphere; how much of those greenhouse gases are absorbed by the ocean, the biosphere, and other sinks; and how sensitive Earth’s climate is to those emissions.23 Climate sensitivity is typically defined as the long-term change that would result from a doubling of carbon dioxide in the atmosphere relative to preindustrial levels; its exact value is uncertain due to the interconnected nature of the land–atmosphere–ocean system. Changes in one aspect of the system can lead to self-reinforcing cycles that can either amplify or weaken the climate system’s responses to human and natural influences, creating a positive feedback or self-reinforcing cycle in the first case and a negative feedback in the second.18 These feedbacks operate on a range of timescales from very short (essentially instantaneous) to very long (centuries). While there are uncertainties associated with modeling some of these feedbacks,24,25 the most up-to-date scientific assessment shows that the net effect of these feedbacks over the industrial era has been to amplify human-induced warming, and this amplification will continue over coming decades18 (see Box 2.3).

Box 2.3: The Climate Science Special Report (CSSR), NCA4 Volume I

This chapter highlights key findings from the Climate Science Special Report (2017).

Periodically taking stock of the current state of knowledge about climate change and putting new weather extremes, changes in sea ice, increases in ocean temperatures, and ocean acidification into context ensures that rigorous, scientific-based information is available to inform dialog and decisions at every level. This is the purpose of the USGCRP’s Climate Science Special Report (CSSR),208 which is Volume I of the Fourth National Climate Assessment (NCA4), as required by the U.S. Global Change Research Act of 1990. CSSR updates scientific understanding of past, current, and future climate change with the observations and research that have emerged since the Third National Climate Assessment (NCA3) was published in May 2014. It discusses climate trends and findings at the global scale, then focuses on specific areas, from observed and projected changes in temperature and precipitation to the importance of human choice in determining our climate future.

Since NCA3, stronger evidence has emerged for continuing, rapid, human-caused warming of the global atmosphere and ocean. The CSSR definitively concludes that, “human activities, especially emissions of greenhouse gases, are the dominant cause of the observed climate changes in the industrial era, especially over the last six decades. Over the last century, there are no credible alternative explanations supported by the full extent of the observational evidence.”

Since 1980, the number of extreme weather-related events per year costing the American people more than one billion dollars per event has increased significantly (accounting for inflation), and the total cost of these extreme events for the United States has exceeded $1.1 trillion. Improved understanding of the frequency and severity of these events in the context of a changing climate is critical.

The last few years have also seen record-breaking, climate-related weather extremes, the three warmest years on record for the globe, and continued decline in arctic sea ice. These types of records are expected to continue to be broken in the future. Significant advances have also been made in the understanding of observed individual extreme weather events, such as the 2011 hot summer in Texas and Oklahoma,209,210,211 the recent California agricultural drought,212,213 the spring 2013 wet season in the Upper Midwest,214,215 and most recently Hurricane Harvey (see Box 2.5),216,217,218 and how they relate to increasing global temperatures and associated climate changes. This chapter presents the highlights from CSSR. More examples are provided in Vose et al. (2017),85 Table 6.3; Easterling et al. (2017),94 Table 7.1; and Wehner et al. (2017),101 Table 8.1; and additional details on what is new since NCA3 can be found in Fahey et al. (2017),18 Box 2.3.

Because it takes some time for Earth’s climate system to fully respond to an increase in greenhouse gas concentrations, even if these concentrations could be stabilized at their current level in the atmosphere, the amount that is already there is projected to result in at least an additional 1.1°F (0.6°C) of warming over this century relative to the last few decades.24,26 If emissions continue, projected changes in global average temperature corresponding to the scenarios used in this assessment (see Box 2.4) range from 4.2°–8.5°F (2.4°–4.7°C) under a higher scenario (RCP8.5) to 0.4°–2.7°F (0.2°–1.5°C) under a very low scenario (RCP2.6) for the period 2080–2099 relative to 1986–2015 (Figure 2.2).24 However, these scenarios do not encompass all possible futures. With significant reductions in emissions of greenhouse gases, the future rise in global average temperature could be limited to 3.6°F (2°C) or less, consistent with the aim of the Paris Agreement (see Box 2.4).27 Similarly, without major reductions in these emissions, the increase in annual average global temperatures relative to preindustrial times could reach 9°F (5°C) or more by the end of this century.24 Because of the slow timescale over which the ocean absorbs heat, warming that results from emissions that occur during this century will leave a multi-millennial legacy, with a substantial fraction of the warming persisting for more than 10,000 years.28,29,30

Box 2.4: Cumulative Carbon and 1.5°/2°C Targets

Limiting global average temperature increase to 3.6°F (2°C) will require a major reduction in emissions.

Projections of future changes in climate are based on scenarios of greenhouse gas emissions and other pollutants from human activities. The primary scenarios used in this assessment are called Representative Concentration Pathways (RCPs)219 and are numbered according to changes in radiative forcing (a measure of the influence that a factor, such as greenhouse gas emissions, has in changing the global balance of incoming and outgoing energy) in 2100 relative to preindustrial conditions: +2.6 (very low), +4.5 (lower), +6.0 (mid-high) and +8.5 (higher) watts per square meter (W/m2). Some scenarios are consistent with increasing dependence on fossil fuels, while others could only be achieved by deliberate actions to reduce emissions (see Section 4.2 in Hayhoe et al. 201724 for more details). The resulting range in forcing scenarios reflects the uncertainty inherent in quantifying human activities and their influence on climate (e.g., Hawkins and Sutton 2009, 201123,220).

Which scenario is more likely? The observed acceleration in carbon emissions over the past 15–20 years has been consistent with the higher future scenarios (such as RCP8.5) considered in this assessment.221,222,223 Since 2014, however, the growth in emission rates of carbon dioxide has begun to slow as economic growth has become less carbon-intensive224,225,226 with the trend in 2016 estimated at near zero.227,228 Preliminary data for 2017, however, indicate growth in carbon emissions once again.228 These latest results highlight how separating systemic change due to decarbonization from short-term variability that is often affected by economic changes remains difficult.

To stabilize the global temperature at any level requires that emission rates decrease eventually to zero. To stabilize global average temperature at or below specific long-term warming targets such as 3.6°F (2°C), or the more ambitious target of 2.7°F (1.5°C), would require substantial reductions in net global carbon emissions relative to present-day values well before 2040, and likely would require net emissions to become zero or possibly negative later in the century. Accounting for emissions of carbon as well as other greenhouse gases and particles that remain in the atmosphere from weeks to centuries, cumulative human-caused carbon emissions since the beginning of the industrial era would likely need to stay below about 800 GtC in order to provide a two-thirds likelihood of preventing 3.6°F (2°C) of warming, implying that approximately only 230 GtC more could be emitted globally in order to meet that target.27 Several recent studies specifically examine remaining emissions commensurate with 3.6°F (2°C) warming. They show estimates of cumulative emissions that are both smaller and larger due to a range of factors and differences in underlying assumptions (e.g., Millar et al. 2017 and correction, Rogelj et al. 2018229,230,231).

If global emissions are consistent with a pathway that lies between the higher RCP8.5 and lower RCP4.5 scenarios, emissions could continue for only about two decades before this cumulative carbon threshold is exceeded. Any further emissions beyond these thresholds would cause global average temperature to overshoot the 2°C warming target. At current emission rates, unless there is a very rapid decarbonization of the world’s energy systems over the next few decades, stabilization at neither target would be remotely possible.27,229,232,233

In addition, the warming and associated climate effects from carbon emissions will persist for decades to millennia.234,235 Climate intervention or geoengineering strategies, such as solar radiation management, are measures that attempt to limit the increase in or reduce global temperature. For many of these proposed strategies, however, the technical feasibilities, costs, risks, co-benefits, and governance challenges remain unproven. It would be necessary to comprehensively assess these strategies before their benefits and risks can be confidently judged.27

Two line graphs show current and projected emissions measured in gigatons of carbon per year and average temperature change relative to 1986 to 2015. The historical line starts just above zero gigatons of carbon per year in 1990 and increases to about 10 gigatons per year in 2015. The higher scenario, RCP8.5, shows a projected increase of nearly 30 gigatons of carbon per year by 2100. The mid-high scenario, RCP6.0, shows an increase to about 15 gigatons of carbon per year. The lower scenario, RCP4.5, shows a decrease to about 5 gigatons per year, and the even lower scenario RCP2.6, shows a decrease below zero gigatons per year by the end of the century.
The second line graph shows the resulting temperature changes under the different emission scenarios, ranging from an increase by 7 degrees Fahrenheit (about 4 degrees Celsius) under the higher emissions scenario, RCP8.5, to an increase in 1.8 degrees Fahrenheit (about 0.9 degrees Celsius) under the very low emissions scenario, RCP2.6, by 2100. The two middle emissions scenarios lead to an increase in temperature by about 3.5 degrees Fahrenheit (2 degrees Celsius) by the end of the century.

Figure 2.2: Observed and projected changes in global average temperature (right) depend on observed and projected emissions of carbon dioxide from fossil fuel combustion (left) and emissions of carbon dioxide and other heat-trapping gases from other human activities, including land use and land-use change. Under a pathway consistent with a higher scenario (RCP8.5), fossil fuel carbon emissions continue to increase throughout the century, and by 2080–2099, global average temperature is projected to increase by 4.2°–8.5°F (2.4°–4.7°C; shown by the burnt orange shaded area) relative to the 1986–2015 average. Under a lower scenario (RCP4.5), fossil fuel carbon emissions peak mid-century then decrease, and global average temperature is projected to increase by 1.7°–4.4°F (0.9°–2.4°C; range not shown on graph) relative to 1986–2015. Under an even lower scenario (RCP2.6), assuming carbon emissions from fossil fuels have already peaked, temperature increases could be limited to 0.4°–2.7°F (0.2°–1.5°C; shown by green shaded area) relative to 1986–2015. Thick lines within shaded areas represent the average of multiple climate models. The shaded ranges illustrate the 5% to 95% confidence intervals for the respective projections. In all RCP scenarios, carbon emissions from land use and land-use change amount to less than 1 GtC by 2020 and fall thereafter. Limiting the rise in global average temperature to less than 2.2°F (1.2°C) relative to 1986–2015 is approximately equivalent to 3.6°F (2°C) or less relative to preindustrial temperatures, consistent with the aim of the Paris Agreement (see Box 2.4). Source: adapted from Wuebbles et al. 2017.10